The technology of cooling practiced most in the art is Joule-Thompson expansion, which is known to produce cooling by the forced mechanical separation of molecules in a jet. A gas or liquid under pressure is released through an expansion valve, and the attractive forces between the molecules are overcome by momentum. The flow is turbulent. This is known to be an inefficient process, but where abundant energy is available, as in jet airplane intercooling, this disadvantage is negligible.
However, for home or industrial refrigeration, energy efficiency is important. Fluids used must be easily torn apart by the Joule-Thompson expansion valve. Fluorocarbons meet this requirement, but have been found to have adverse environmental effects. Using only one phase is practiced currently in the art of vapor compression refrigeration. That phase is fluorocarbons due to their superior qualities when used in a Joule-Thompson expansion valve. However, fluorocarbons are known to be a danger to the environment and are scheduled for extinction soon. Two-phase systems known to the art are bulky brine systems and adsorption devices.
Vapor compression refrigeration cycles known to the art comprise two stages. In the first stage, vapor of the refrigerant is compressed, liberating the latent heat of the refrigerant vapor. The vapor usually is compressed to the point of condensation, although gas refrigeration cycles that do not change state are known. The condensate or compressed gas is pushed through a long conduit to engage in heat exchange with the ambient fluid, generally air, so as to discharge the latent heat from the system. The conduit is generally a narrow pipe to maximize the heat exchange surface. Friction loss from pumping liquid through a narrow pipe makes this an inefficient system.
Screw or centrifugal compressors, where compression is accomplished by centrifugal force pressing the refrigerant vapor against a wall, have been used in large applications. Centrifugal pumps are unable to produce the high head needed for pushing condensed refrigerant through the long and narrow pipe of a small refrigerator or air conditioner heat exchange section, so generally positive displacement pumps are used. The pressure in the conduit is greater than the pressure of the atmosphere so that no air or water vapor can intrude into the system. Oil from the seals of these positive displacement pumps can contaminate the refrigerant, resulting in loss of efficiency.
After its passage through the heat exchange section, the high-pressure cooled refrigerant condensate is then released through an expansion valve into another tube, beginning the second stage, which is where cooling actually takes place. The lowering of pressure allows evaporation. Evaporation draws heat from the walls of the tube, which in turn draw heat from the ambient air around the food or other item to be cooled. Once evaporated, the refrigerant is recondensed in the first stage, renewing the cycle.
Pushing condensate through long, narrow, high-pressure conduits by positive displacement pumps requires an inordinate amount of energy. Furthermore, positive displacement pumps hammer the condensate, causing constant vibration of the long, narrow, high pressure conduits, resulting in fatigue in the materials and leaks of refrigerant through cracks.
The use of CFCs (chloroflourocarbons) and other dangerous refrigerants in such vulnerable circuits is a matter of increasing concern. CFCs have been found to damage the ozone layer of the atmosphere, and the production of CFCs after the year 2000 has been banned by Title VI of the Clean Air Act Amendments of 1990, Pub. L. No. 101-549, 104 Stat. 2399 (1990). A new refrigeration method and apparatus is especially needed for automobile air conditioning units because in 1994 a phase-in is to begin that will preclude the sale of automobiles containing ozone-depleting refrigerants. 42 U.S.C. Section 7671 h.
Ammonia is used in industrial chillers. Its disadantage is that it is explosive and poisonous.
Two-phase refrigeration systems cool a fluid and then circulate that cooled fluid to engage in heat exchange with the material to be cooled. The cooled fluid is known as the secondary refrigerant. It does not change state during the refrigeration process, but merely acts as a heat exchange medium. For example, brine is used as a secondary refrigerant in ammonia refrigeration systems for making ice. The brine does not mix with ammonia and does not change state; it merely acts as a medium for drawing heat out of the water which is turned into ice.